In typical variable pitch propulsor systems, a plurality of propulsor blades, each rotatably mounted for movement about its longitudinal axis to a rotary hub driven by the aircraft engine, are operatively connected to a mechanical or hydromechanical blade pitch change actuation system disposed in an interior chamber defined within the hub. As well recognized by those skilled in the art, it is necessary in such blade pitch change actuation systems to provide a pitchlock for maintaining blade pitch in the event of a malfunction in the system's hydraulic supply and also an end of stroke stop to prevent the blades from inadvertently going to a coarse pitch setting outside of the operating range.
An example of a hydromechanical blade pitch change actuation system that is equipped with a mechanical pitchlock and an end of stroke stop is presented is commonly assigned U.S. Pat. No. 4,523,891, to Schwartz and Duchesneau. In the pitch change actuation system disclosed therein, commonly known as a linear hydromechanical actuation system, pitchlock is provided by maintaining a relatively small pitchlock gap between the forward end of a pitchlock screw threadably mounted within a bulkhead of the pitch change actuation piston and a stop extending from the hub wall into its interior. At any pitch setting, the pitchlock screw is positioned relative to the pitch stop such that the pitch change actuation piston cannot translate toward fine pitch without the pitchlock screw quickly traversing the relatively small pitchlock gap and grounding against the pitch stop thereby precluding any further movement of the actuation piston towards fine pitch.
Passage through feather is precluded in this system by means of a radially directed stud which extends outwardly from an aft portion of the pitchlock screw and a cooperating feather stop which extends axially from the central bulkhead of the piston through which the pitchlock screw is threaded. As the pitch change actuation piston translates rearwardly towards maximum coarse pitch, the feather stop moves rearwardly therewith until it is engaged by the stud on the rotating pitchlock screw. Upon such engagement, further rotation of the pitchlock screw is physically impossible and further translation of the pitch change actuation piston is consequently precluded, thus preventing jamming of the pitchlock screw when the actuator has reached the end of its stroke and placed the propeller blades in feather.
Conventionally, propeller blades are passed through low pitch when it is desired to set the blades to a desired reverse pitch. A hydromechanical pitch control system having a fine pitch stop which may be released to permit the propeller blades to pass through low pitch to reverse pitch is disclosed in commonly-assigned U.S. Pat. No. 3,212,586. In the pitch control system disclosed therein, pivoting stop fingers engage lugs associated with the pitch change actuator to limit fine pitch movement of the blades to a preselected low pitch setting during normal operation. The pivoting fingers are operatively connected to a spring loaded, hydraulic piston which may be selectively actuated via hydraulic fluid pressure to translate against the spring force so as to pivot the stop fingers out of engagement with the pitch change actuator, thereby permitting the actuator to drive the blades through the low pitch limit setting to a desired reverse blade pitch.
While performing its functions satisfactorily on such conventional propeller systems, such a pitch change actuator would be unsuitable for use on a system wherein blade pitch is reversed by passing the blades through feather, rather than flat pitch, such as required on variable pitch propulsors having a large number of blades, such as state-of-the-art turbofans being considered for use on turbine driven aircraft, since the feather stop prevents the actuator from passing the blades through feather.